Low temperature air cooling is needed to maintain low dew point temperatures in spaces such as hospital operating rooms, archival storage, indoor ice rinks, supermarkets etc . . .
Mechanical cooling and dehumidification systems are generally restricted to delivering air temperatures safely above freezing to avoid complications associated with frost buildup in the cooling coil and subsequent air restriction.
Cooling coils typically comprise tubes, through which flows a coolant such as water, brine or a refrigerant. The inner surface of the tubes can have enhancements to improve heat transfer between the coolant and the tube. Air, flows over the outside of the tubes where fins can be added to enhance heat transfer between the tube and air. The coolant inside the tubes removes heat from the air and thus cools the air.
Frost formation begins during operation when coolant temperature is below freezing and air temperature reaches saturation. Frost accumulates over time and eventually restricts airflow, requiring some means for defrosting.
Defrosting may employ introduction of hot gas, from a refrigeration plant, injected into the tubes, thereby defrosting from the inside out. This introduction of heat to a cooling process interrupts operation and is inefficient, resulting in an increase in overall energy consumption.